AppDelegate, rootViewController and presentViewController

I had the same issue. Based on the answer to this question, I added [self.window makeKeyAndVisible] just before presentViewController:animated:completion:, and that fixed it for me.

In your case, showLoginView becomes

- (void)showLoginView
{
    UIStoryboard *storyboard = [UIStoryboard storyboardWithName:@"MainStoryboard" bundle:nil];
    LoginViewController *loginViewController = [storyboard instantiateViewControllerWithIdentifier:@"LoginViewController"];
    [self.window makeKeyAndVisible];
    [self.window.rootViewController presentViewController:loginViewController animated:YES completion:NULL];
}

Sometimes presenting modal view controller from window.rootViewController may produce the same warning & have no effect. Example of such hierarchy of view controllers:

1. [MYUITableViewController] (presented modally by MYUIViewController )
2. [MYUIViewController] (rootViewController of UINavigationController below)
3. [UINavigationController] (root)

Now calling

[[UIApplication sharedApplication].keyWindow.rootViewController presentViewController:[UIViewController new] animated:YES completion:nil];

will cause this exact warning (tested both on iOS6 & 7 Sim)

Solution: Instead of using rootViewController - use the top one presented by it:

    UIViewController *topRootViewController = [UIApplication sharedApplication].keyWindow.rootViewController;
    while (topRootViewController.presentedViewController) 
    {
        topRootViewController = topRootViewController.presentedViewController;
    }

    [topRootViewController presentViewController:yourController animated:YES completion:nil];

• Sometimes keyWindow may have been replaced by window with nil rootViewController (showing UIAlertViews, UIActionSheets on iPhone, etc), in that case you should use UIView's window property.